1. Field of the Invention
This invention relates to an optical address type spatial light modulator.
2. Description of the Related Art
Application of an optical address type spatial light modulator to an optical amplification element for projection type display, an optical computation element for optical computing, or a display element with the optical address type spatial light modulator itself as a medium, etc., is examined.
FIG. 1 is a drawing to show an example of an optical address type spatial light modulator and its write section previously used.
An optical address type spatial light modulator 1 in FIG. 1 is made up of a pair of substrates 17 and 18 formed on inner faces with electrodes 19 and 20 and a liquid crystal layer 21 for reflecting incident read light 29, a photoconductive layer 22 with impedance changing depending on incident write light 28, and a light separation layer 23 being placed between the liquid crystal layer 21 and the photoconductive layer 22 for preventing leakage of the read light 29 to the side of the photoconductive layer 22 and leakage of the write light 28 to the side of the liquid crystal layer 21, the layers 21, 22, and 23 being sandwiched between the substrates 17 and 18. As the liquid crystal layer 21, any of various liquid crystal elements different in optical effect, such as a technique of using polarization state change of homeotropic-aligned nematic liquid crystal, homogeneous-aligned nematic liquid crystal, twisted nematic liquid crystal, supertwisted nematic liquid crystal, surface stabilized ferroelectric liquid crystal, etc., a technique of using light scattering state change of polymer dispersed liquid crystal, etc., a technique of using light absorption state change of guest host liquid crystal, etc., or a technique of using optical interference state change of cholesteric (chiral nematic) liquid crystal, etc., can be used. As the photoconductive layer 22, an element having an internal photoelectric effect produced by the write light 28, such as an inorganic photoconductive film of a—Si:H, CdS, etc., or an organic photoconductive film provided by combining a charge generation layer consisting of azo pigment, phthalocyanine pigment, etc., and a charge transport layer consisting of hydrazone, aryl amine, etc., is used. As the light separation layer 23, a dielectric mirror comprising substances different in refractive index such as TiO2 and SiO2 deposited alternately for interference-reflecting the write light 28 is used and a light absorption layer is provided for absorbing the write light 28 between the dielectric mirror and the photoconductive layer 22 as required when the write light 28 is strong, etc. In the technique of using optical interference state change, only the light absorption layer is used as the light separation layer 23.
A write section 2 comprises a voltage application section 24 with a power supply 27 connected to the electrodes 19 and 20 of the optical address type spatial light modulator 1 for applying a predetermined voltage, a light application section 26 for applying the write light 28 to the photoconductive layer 22, and a control section 25 for controlling the timings, etc., of applying the voltage from the voltage application section 24 and applying the write light from the light application section 26.
FIG. 2 is an equivalent circuit diagram of the optical address type spatial light modulator.
In FIG. 2, the optical address type spatial light modulator is represented as a circuit wherein the liquid crystal layer 21, the photoconductive layer 22, or the light separation layer 23 that can be replaced as a parallel circuit of a resistor and a capacitance and the electrode 19, 20 that can be replaced as a resistor are connected in series, and bias voltage V applied between the electrodes 19 and 20 from the write section 2 is divided by the impedance of each circuit. When the write light is applied from the light application section 26 to the photoconductive layer 22, a resistance value R4 of the photoconductive layer 22 lowers and thus division voltage V2 applied to the portion of the liquid crystal layer 21 to which the write light is applied becomes higher than the portion to which the write light is not applied. Therefore, the voltage distribution of the liquid crystal layer 21 changes with the light intensity of the write light and the optical state of the liquid crystal layer 21 also changes in response to the voltage distribution, so that the light intensity distribution of the write light can be reflected on the reflectivity distribution of the read light.
The optical address type spatial light modulator 1 previously used, shown in FIG. 1 can change the reflection strength of the read light 29, but cannot change the wavelength distribution of the read light 29. Therefore, for example, to use the optical address type spatial light modulator 1 as an optical amplification element for projection type display capable of producing color display, dichroic mirrors for reflecting light in response to the wavelength are used.
FIG. 3 is a drawing to show an example of optical address type spatial light modulators using dichroic mirrors.
As shown in FIG. 3, dichroic mirrors 35 and 36 are used to separate incident read light 32 into a plurality of read light beams different in wavelength, for example, R (red) light, G (green) light, and B (blue) light, and mirrors 33 and 34, etc., are used to make the R read light, the G read light, and the B read light incident on separate optical address type spatial light modulators 30A, 30B, and 30C. On the other hand, write light 31 is also separated into R light, G light, and B light by dichroic mirrors 37 and 38 so as to correspond to the R read light, the G read light, and the B read light, and mirrors 39 and 40 are used to apply the R light, the G light, and the B light to the optical address type spatial light modulators. The R read light, the G read light, and the B read light strength-modulated in response to the light intensities of the R write light, the G write light, and the B write light are again combined and are observed as one read light 32. Thus, the optical address type spatial light modulators are provided for changing the wavelength distribution of the read light by using the method of separating the read light 32 and the write light 31 into color light beams different in wavelength. However, the optical address type spatial light modulators thus configured require a complicated optical system and high registration accuracy and thus involve problems of a high apparatus cost and a large apparatus size. Further, separate optical address type spatial light modulators are required in a one-to-one correspondence with the wavelength bands of read light and the incidence and reflection directions of read light are limited and thus it is difficult to use a single optical modulator as a display element for modulating outside light.
It is therefore an object of the invention to provide an optical address type spatial light modulator which makes it possible to change the wavelength distribution of read light according to a simple structure and can also be used as a display element for modulating outside light.